1. Field of the Invention
The present invention relates to a method of annealing a semiconductor film with laser beams (hereinbelow referred to as laser annealing) and a laser apparatus to be used for performing the same (more specifically, an apparatus including a laser source and an optical system for guiding laser beams emitted from the laser source to an object to be processed).
2. Description of the Related Art
In recent years, developments of thin film transistors (hereinbelow referred to as TFTs) has been advanced, and in particular, TFTs employing a polycrystalline silicon film (polysilicon film) as a crystalline semiconductor film has drawn much attention. Especially, in a liquid crystal display device (liquid crystal display) or an EL (electro-luminescence) display device (EL display), these TFTs are used as elements for switching pixels and elements constituting a driver circuit for controlling the pixels.
In common techniques for obtaining a polysilicon film, an amorphous silicon film) is crystallized to obtain a polysilicon film. In particular, a method of crystallizing an amorphous silicon film with laser beams has been receiving much attention. In the present specification, the technique for crystallizing an amorphous semiconductor film with laser beams to obtain a crystalline semiconductor film is referred to as laser crystallization.
The laser crystallization enables instantaneous heating of a semiconductor film, and thus it is an effective technique for annealing a semiconductor film formed on a substrate having low heat-resistance, such as a glass substrate, a plastic substrate or the like. In addition, the laser crystallization has a significantly higher throughput, as compared to conventional heating means employing an electric furnace (hereinbelow ref erred to as furnace annealing).
Although various kinds of laser beams are available, laser beams emitted from a pulse-oscillated excimer laser (hereinbelow referred to as excimer laser beams) are generally used in the laser crystallization. The excimer laser can provide a large output power and repeat irradiation at high frequencies. Furthermore, the excimer laser beams have an advantage of a high absorption coefficient against a silicon film.
One of the most important problems to be solved in these days is how to enlarge the diameters of crystal grains in a crystalline semiconductor film crystallized with laser beams. It is clear that the larger each crystal grain (also simply referred to as a grain) is, the less number of grains traverse TFTs, in particular. channel-formation regions thereof. This enables improvements in the fluctuation of typical electrical characteristics of TFTs, such as a field effect mobility or a threshold voltage.
In addition, since relatively satisfactory crystallinity is maintained at the inside of each grain, it is desirable, when fabricating TFTs, to dispose the entire channel-formation region within a single grain so as to improve the above-mentioned various operational characteristics of TFTs.
However, it is difficult to obtain a crystalline semiconductor film having sufficiently large grain diameters with employment of presently available techniques. Although some results have been reported indicating that such a crystalline semiconductor film with sufficiently large grain diameters was experimentally obtained those reported techniques have not reached practical levels yet.
For example, in the experimental level, the results have been achieved as described in the article entitled xe2x80x9cHigh-mobility poly-Si thin-film transistors fabricated by a novel excimer laser crystallization methodxe2x80x9d by K. Shimizu, O. Sugiura and M. Matsumura in IEEE Transactions on Electron Devices, vol. 40, No. 1, pp. 112-117 (1993). In this article, a three-layered structure of Si/SiO2/n+Si is formed on a substrate, and this layered structure is irradiated with excimer laser beams from both the Si side and the n+Si side. The article explained that larger grain diameters can be thus obtained.
The present invention is intended to overcome the above-mentioned disadvantages In the art by providing a laser annealing method capable of providing a crystalline semiconductor film with larger grain diameters, and a laser apparatus to be used in such a laser annealing method.
In accordance with the present invention, upon crystallization of an amorphous semiconductor film, a top surface (on which a thin film is to be deposited) and a back surface (a surface opposite to the top surface) of the amorphous semiconductor film are simultaneously irradiated with laser beams while an effective energy intensity of the laser beams to be applied onto the top surface (hereinafter referred to as first laser beams) is set at a level different from that of the laser beams to be applied onto the back surface (hereinafter referred to as second laser beams).
More specifically, the irradiation conditions of the laser beams are set so that the effective energy intensity ratio Ioxe2x80x2/Io between the effective energy intensity Io of the first laser beams and the effective energy intensity Ioxe2x80x2 of the second laser beams satisfies the relationship of 0 less than Ioxe2x80x2/Io less than 1 or 1 less than Ioxe2x80x2/Io, where the product of Io and Ioxe2x80x2 (Ioxc3x97Ioxe2x80x2) is not equal to zero.
In the present specification, the term xe2x80x9ceffective energy intensityxe2x80x9d is defined as the energy intensity of the laser beams at the top or back surface of an amorphous semiconductor film while taking energy losses, caused by various reasons such as reflection or the like, into consideration. The unit of the effective energy intensity is the same as that of the energy density, i.e., mJ/cm2. Although the effective energy intensity can not be directly measured, it can be calculated based on known parameters such as a reflectance or a transmittance as long as medium present along a path of the laser beams is known.
For example, the calculation method of the effective energy intensity will be described in more detail by taking as an example the case where the present invention is applied to the structure as illustrated in FIG. 6. In FIG. 6, reference numeral 601 denotes a reflector made of aluminum, 602 denotes a Corning #1737 substrate (having a thickness of 0.7 mm), 603 denotes a silicon oxynitride film (hereinbelow referred to as SiON film) having a thickness of 200 nm, and 604 denotes an amorphous silicon film having a thickness of 55 nm. This sample is irradiated with XeCl excimer laser beams having a wavelength of 308 nm in air.
The energy intensity of the excimer laser beams (with the wavelength of 308 nm) immediately before reaching the amorphous silicon film 604 is represented as Ia. Taking into consideration the reflection of the laser beams at the surface of the amorphous silicon film, the effective energy intensity Io of the first laser beams can be expressed as Io=Iaxc3x97(1xe2x88x92RSi), where RSi indicates the reflectance of the laser beams. In this case, Io is calculated as 0.45xc3x97Ia.
Furthermore, the effective energy intensity Ioxe2x80x2 of the second laser beams can be expressed as Ioxe2x80x2=Iaxc3x97T1737xc3x97RAlxc3x97T1737xc3x97(1xe2x88x92RSiON-Si) where T1737 indicates the transmittance of the #1737 substrate, RAl indicates the reflectance at the aluminum surface, and RSiON-Si indicates the reflectance experienced by the light beams incident onto the amorphous silicon film from the SiON film. The reflectance experienced by the light beams incident onto the SiON film from air, the transmittance in the SiON film, the reflectance experienced by the light beams incident on the #1737 substrate from the SiON film, and the reflectance experienced by the light beams incident on the SiON film from the #1737 substrate are found to be negligible from the experimental results, and therefore not considered in the calculation. In this case, Ioxe2x80x2 is calculated as 0.13xc3x97Ia.
Accordingly, in the structure illustrated in FIG. 6, the effective energy intensity Io of the first laser beams and the effective energy intensity Ioxe2x80x2 of the second laser beams can be calculated as 0.45Ia and 0.13Ia, respectively. Thus, the effective energy intensity ratio Ioxe2x80x2/Io can be calculated as 0.29. The fact that the effective energy intensity ratio thus calculated satisfies the relationship of 0 less than Ioxe2x80x2/Io less than 1 is one of the features of the present invention.
Furthermore, the present invention is applicable to the case where the intensity of the first laser beams is smaller than that of the second laser beams. In other words, the present invention is applicable to the case where the relationship of 1 less than Ioxe2x80x2/Io is satisfied.
The effective energy intensities of the first and second laser beams can be set at different levels by, for example, the following manners:
1) When the top and back surfaces of an amorphous semiconductor film are irradiated with laser beams by means of ea reflector disposed below a substrate, the effective energy intensity of the second laser beams is attenuated by adjusting the reflectance of the reflector so as to become smaller as compared to the effective energy intensity of the first laser beams.
2) The first laser beams are divided to form the second laser beams, and either the effective energy intensity of the first laser beams or that of the second laser beams is attenuated by means of an appropriate filter (such as a variable attenuator or the like) so that the effective energy intensities of the first and second laser beams are set at different levels from each other.
3) The effective energy intensity of the second laser beams is attenuated in accordance with a material of the substrate on which an amorphous semiconductor film is to be deposited, so as to become smaller as compared to the effective energy intensity of the first laser beams.
4) An insulating film is provided between the substrate and the amorphous semiconductor film so that the effective energy intensity of the second laser beams is attenuated by the insulating film, thereby resulting in the smaller effective energy intensity as compared to the effective energy intensity of the first laser beams.
5) The surface of the amorphous semiconductor film is covered with an insulating film so that the reflectance of the first laser beams at the surface of the amorphous semiconductor film becomes smaller, thereby resulting in the effective energy intensity of the first laser beams being larger as compared to that of the second laser beams.
6) The surface of the amorphous semiconductor film is covered with an insulating film so that the effective energy intensity of the first laser beams is attenuated by the insulating film, thereby resulting in the smaller effective energy intensity as compared to the effective energy intensity of the second laser beams.
7) The first and second laser beams are emitted from different oscillating sources, respectively, so that the effective energy intensities of both laser beams are set at different levels.
It should be noted that the present invention is not limited to a specific type of lasers. Rather, various lasers can be used in the present invention: for example, generally known excimer lasers (typically a KrF laser or a XeCl laser), solid-state lasers (typically a Nd:YAG laser or a ruby laser), gas lasers (typically an Ar laser or a Hexe2x80x94Ne laser), metal vapor lasers (typically a Cu vapor laser or a Hexe2x80x94Cd laser), or semiconductor lasers.
In the case where the laser such as Nd:YAG laser which has a fundamental wave (the first harmonic wave) of a long wavelength (1064 nm for Nd:YAG laser) is used, it is preferable to use the second, third or fourth harmonic wave. These high-order harmonic wave can be obtained by means of non-linear crystal (non-linear device). Alternatively, a well-known Q-switch may be used to obtain the higher-order harmonic wave.